This invention relates to permanent magnet electric machines, and more particularly to permanent magnet electric machines having two or more axial rotor sections that are rotationally offset.
Permanent magnet electric machines, such as motors and generators, include a stationary stator that defines salient poles and inter-polar slots that are located between the salient poles. The stator is often mounted on an inner surface of a machine housing with the salient poles projecting radially inwardly. The permanent magnet electric machines also include a rotor that is mounted on a shaft, that includes rotor poles and that rotates on the shaft relative to the stator. The rotor poles include permanent magnets that are attached to a radially outer surface of the rotor. Winding wire is wound around the stator poles in the inter-polar stator slots. A circuit board or another connection device couples the stator pole windings to a drive circuit. The drive circuit generates a set of stator winding currents that are output to the stator pole windings and that result in a rotating magnetic field. The rotating magnetic field in the stator poles interacts with the magnetic poles of the rotor to cause the rotor to rotate.
Electric machines with permanent magnet rotors often have cogging torque that adversely impacts machine performance. Cogging torque is caused by the variation of magnetic permeance as seen by a rotor pole as it passes the stator poles and the slot openings. Cogging torque occurs when the stator windings are un-energized. The rotor seeks a rotational position that results in the lowest magnetic circuit reluctance (or the highest permeance). The rotational position with the lowest magnetic circuit reluctance occurs when a rotor pole is aligned with a stator pole. When the rotor pole is aligned with a slot opening, the rotor pole will attempt to align itself with a stator pole, thereby producing torque. The cogging torque oscillates between positive and negative torque, depending on the position of the rotor poles with respect to the stator poles. The torque oscillations cause vibration and noise within the permanent magnet electric machine. The variation in torque can also cause vibration in the equipment that is driven by the machine, which causes additional noise.
Various methods for reducing cogging torque have been proposed. In one method, the permanent magnets are skewed in an angled pattern or in a herringbone pattern on the outer surface of the rotor. Skewing the permanent magnets increases material and manufacturing costs due to the complex and non-uniform shape of the permanent magnets. The non-uniform permanent magnets are also difficult to assemble.
Therefore, a permanent magnet electric machine that significantly reduces cogging torque and that can be assembled relatively easily and with relatively low manufacturing costs would be desirable.
A permanent magnet electric machine according to the invention includes a plurality of axial rotor sections that are defined on a radially outer surface of a rotor. The axial rotor sections include a set of permanent magnets that are in an unmagnetized state and that have opposite edges that are aligned with an axis of the rotor. The axial rotor sections are rotationally offset such that the edges of the permanent magnets create interfaces. The permanent magnets are magnetized using a magnetizing fixture.
In other features of the invention, the permanent magnets have a generally rectangular shape and are preferably one of arc magnets and breadloaf magnets. A first offset angle of the axial rotor sections is substantially equal to 360 mechanical degrees divided by a least common multiple of a first number of stator slots of the machine and a second number of rotor poles of the rotor, and divided by a third number of axial rotor sections.
In still other features of the invention, the sets of permanent magnets include m magnet poles and the magnetizing fixture includes at least m slots for conductors. A skew angle of the magnetizing fixture is substantially equal to 360 mechanical degrees divided by the least common multiple of the first number and the second number, multiplied by a stack length of the magnetizing fixture, and divided by a stack length of the rotor.
In still other features, the m conductor slots of the magnetizing fixture are aligned with the interfaces during magnetization. A magnetic field impressed upon the permanent magnets is substantially reduced along the stair step interfaces.
Other objects, features and advantages will be apparent from the specification, the drawings and the claims that follow.